Abstract: Solar energy and air source hybrid heat pump has different performance in different modes. How to operate in the best working mode in practical application is very important for improving the comprehensive performance of the system. Therefore,two judgment methods for realizing the optimal mode switching of the system are proposed:1)The composite environmental parameter range table of solar radiation intensity and ambient temperature based on the optimal mode switching;2)Refrigerant mass flow distribution ratio through two evaporators（solar collector/evaporator and air-cooled evaporator）. Based on this,the simulation model of solar/air dual source auxiliary heat pump water heater is established in Engineering Equation Solrer（EES）. Firstly,the system performance under different modes is simulated and compared based on the climate conditions in Songjiang District,Shanghai,and then the feasibility of taking the composite environmental parameter range table and mass flow distribution ratio as the judgment method of system optimal mode switching is analyzed and discussed. The results show that at the same ambient temperature,with the increase of solar radiation intensity,the optimal operation mode of the system will change from solar-air mode to solar mode. In addition,it is feasible to take the refrigerant mass flow distribution ratio flowing through the two evaporators as the criterion for mode switching. For the system,the critical value of the mass flow distribution ratio when the optimal operation mode changes from solar-air mode to solar mode is about 2.02.